The invention relates to a method of removing foreign matter from a digital hydraulic pressure controller of a hydraulic system and, more exactly speaking, to a method of removing foreign matter from a digital hydraulic pressure controller of a hydraulic system of a machine for producing a fibrous material web, especially a paper or board machine.
In paper machines, hydraulics is widely used as means of operation and control; in particular, actuators by means of which great forces can be adjusted and exerted with high precision are hydraulically driven.
Normally a working fluid, e.g. hydraulic oil, which is pressurized by a pump, is used. The introduction of the pressurized hydraulic oil into a hydraulic actuator, such as a hydraulic cylinder or a hydraulic motor, is typically controlled by a proportional control valve or a proportional valve which can be driven electrically, hydraulically or pneumatically.
Such a control valve has a movable or displaceable spool valve or control piston which, in response to its position within an associated valve housing, can adjust a target pressure at the output by regulating down the pressure of the hydraulic oil supplied by the pump. The mobility of the control piston in the valve housing requires a certain play or clearance between the control piston and the valve housing so that inner leakage of the control valve is unavoidable. The clearance must not be selected to be too narrow, since otherwise the valve would be too prone to contamination in the hydraulic oil.
Recently, alternative pressure controllers have been developed which shall consistently be referred to as digital hydraulic pressure controllers in the present application.
The mode of operation of the digital hydraulic pressure controllers is widely known already. For the sake of improved readability of the present application, however, the mode of operation of digital hydraulic pressure controllers is briefly summarized hereinafter:
In the simple case, a digital hydraulic pressure controller consists of a row of valves which are switched in parallel and which merely have an ON/OFF function; i.e. they are simple ON/OFF switching valves which permit or interrupt a flow and can consistently be referred to as valves in the present application. All of the valves are, on the one hand, connected to a common supply line and, on the other hand, to a common output line. The valves themselves can be conventional solenoid valves, i.e. valves having an electromagnetic drive. Of course, other drive forms may also be selected.
By connecting or installing throttle elements or by the valves themselves it is ensured that the valves have different flow cross-sections and thus different flows when they are opened; a throttle element together with a valve constitutes a valve means. If, for example, four valves are provided, the flow rates Q in the individual flow cross-sections each of which is selectively openable by the associated valve can be at a ratio of 1:2:4:8 with respect to each other; in the case of a larger number of valves, this row is continued accordingly.
By opening and closing individual valves or valve combinations which are determined and selected by a computer on the basis of mathematical models, a very rapid and precise pressure adjustment in the output line or in the actuator connected thereto can be achieved. This is accomplished by replacing the analog control curve of the proportional control valve described above by a digitally generated (approximated) control curve. Due to the omission of non-linearities and/or hysteresis of the analog proportional valve, this curve may be a straight line which is approximated stepwise and allows a set point to be approached quickly and (almost) free from overshoot.
It is another advantage of the digital hydraulic control that the valves are either open or closed, i.e. the valves are simply closed for maintaining a target pressure within a closed (and unchanged) system and there are no internal leakage flows. Thus a clear difference from the conventional proportional valve is given through which constantly a hydraulic oil flow is passed. This requires continuously energy for the hydraulic pumps, e.g. in the paper machine.
Consequently, it is evident that the use of digital hydraulic pressure controllers allows operating the hydraulic pumps less frequently and for a shorter time, whereby energy can be saved.
When operating a digital hydraulic control as described in the foregoing, it may happen that foreign matter occurs in the valves and/or in the common lines which may disturb a smooth operation of the control. Such foreign matter can get into the system, inter alia, when exchanging one or more of the valves during maintenance. Also air or air bubbles are referred to as foreign matter which may occur in the system when such digital hydraulic system is taken into operation for the first time. Foreign matter of this type must be removed or flushed out of the system for a faultless operation of the same.